The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 (Stemme).
There are various known types of thermal ink jet (bubblejet) printhead devices. Two typical devices of this type, one made by Hewlett Packard and the other by Canon, have ink ejection nozzles and chambers for storing ink adjacent the nozzles. Each chamber is covered by a so-called nozzle plate, which is a separately fabricated item and which is mechanically secured to the walls of the chamber. In certain prior art devices, the top plate is made of Kapton™ which is a Dupont trade name for a polyimide film, which has been laser-drilled to form the nozzles. These devices also include heater elements in thermal contact with ink that is disposed adjacent the nozzles, for heating the ink thereby forming gas bubbles in the ink. The gas bubbles generate pressures in the ink causing ink drops to be ejected through the nozzles.
The heater elements are embedded in the printhead substrate and covered for protection from the corrosive environment they create in the chamber. High temperatures and frequently vaporizing ink (typically water based) will quickly oxidate most heater materials. The severe hydraulic forces from the cavitation of collapsing bubbles is also highly corrosive to the heater elements. To prevent the heaters from failing prematurely, they are covered with a protective barrier such as tantalum pentoxide.
The pulse of electrical energy sent to each heater element needs to vaporize the ink by heating it through the protective layer(s). The pulse duration should be as short as possible to maximize print speed. This requires the power of the drive pulse to be relatively high. Unfortunately, high voltage drive pulses require larger drive transistors. High voltages also create the potential for arcing and other breakdown mechanisms. For example, the heater must be electrically insulated from the ink because a high voltage causes electrolytic breakdown of the materials in the ink and chamber.
The voltages can be reduced and the current increased to give a pulse of the same power, but high currents induce electromigration above a certain threshold (electromigration is an atomic migration away from points if high current density that eventually thins and ultimately fractures the heater element), higher parasitic resistances (in the heater contacts etc), the drive transistor operates less efficiently and the ‘ground bounce’ is more severe. Ground bounce is inductance in the common ground connection of the heater elements. This can keep the drain voltage from the drive transistor bouncing above zero, or ground. In the extreme, ground bounce can make the drive logic read the voltage at the drain as ‘1’instead of ‘0’.
In light of the problems associated with high current drive pulses, the drive pulses have a voltage of at least 12 Volts and the large drive transistors they require are tolerated.
It is an object of the present invention to provide a useful alternative to the known printheads, printer systems, or methods of ejecting drops of ink and other related liquids, which have advantages as described herein.